Halide Composition Controls Phase Morphology and Device Specific Performance in Perovskite Photovoltaics and Light Emitting Diodes

ABSTRACT Wide‐bandgap mixed‐halide perovskites have attracted significant interest due to their compositional tunability and suitability for a wide range of applications. However, their phase instability remains major challenge as phase segregation process occurs under illumination. The ratio of Br to I affects the stability of the perovskite and the phase segregation process, and when the composition reaches the miscibility gap, two separated phases form instead of a homogeneous mixed phase. Here, we investigate two mixed‐halide perovskite compositions containing 75% Br and 95% Br and evaluate their performance in Light emitting diode and solar cells. Optical characterization reveals that the 95% Br composition lies above the miscibility gap and forms two stable phases, whereas the 75% Br composition forms a single phase that undergoes light‐induced phase segregation. For that reason, their performance mechanism in devices differs, the phase segregated perovskite performed better in solar cells, while the two separated phases exhibit stable performance in Light Emitting Diodes. Our findings provide fundamental insight into how halide composition controls device physics and optoelectronic performance. Moreover, this work highlights that phase heterogeneity should not be viewed solely as a degradation mechanism but rather as a design parameter whose impact depends on the targeted application.